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PICList Thread
'Piezo Buzzer for debugging?'
1998\05\10@083532 by paulb

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Jonathan wrote:

> So I stuck a piezio buzzer on PORTA.  It works as a good in-circuit
> debug, but it is rather high overhead (ie: set the port pin, delay for
> 400ms, clear the port pin, delay for 400ms, repeat for more buzzes).

 It certainly does sound as if he is using a piezo "sounder" with an
internal self-excited oscillator, by which I mean it is a three-terminal
disc providing feedback to a (single) transistor.

>  The buzzer seems to have some odd characteristics also (I don't know
> much about piezo buzzers-- startup time seems a little long, etc).

 This characteristic is *typical* of self-excited oscillators with a
very high "Q" of the disc.  OTOH, they are *very* efficient (noisy).
The "magnetic" type of buzzer (coil; magnet fixed to reed; plastic
diaphragm) using the same principle is also very slow to excite, maybe
just a little faster if you feed it plenty of power (and they consume
*much* more current).

 For these reasons, these devices are quite poor for Morse code.  A
separate electronic oscillator, particularly of the "R-C" or
"relaxation" type rather than the various "tuned curcuit" (State
Variable, Wien, Twin-T) can be started much quicker, or a continuously-
running oscilaltor can be gated to your transducer.

 But this isn't what was wanted.  Point is, you get the PIC to generate
both the "carrier", say 2kHz and gate it with the "modulation", say
1.25 Hz as suggested, and feed that to the transducer.  Unless you are
very lucky (or tune the latter!), it won't match the piezo disc
resonance, but it certainly will go on and off smartly.

 As such, it (or at least the continuous-wave 2kHz version) is a far
better diagnostic than a LED because the ear is *far* better at picking
interruptions and characterising brief transients than the eye.

 On another list I mentioned the virtues of an old-fashioned "crystal"
earpiece as diagnostic tool.  I'll try and bring that up on my webpages
sometime!

 Cheers,
       Paul B.

1998\05\10@122414 by Jonathan

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       Ok, I just ripped apart the 'buzzer' and it is not a piezio
element at all-- it is electromechanical.  (Two wires to the box,
inside the box is a small signal transistor, a couple diodes, a
fat coil, and a spring.)  No wonder it wasn't working well.

       I guess I'll pick up a real piezio buzzer and hope it works
better.  As I want the thing for debugging, it would be very nice to be
able to hear 50ms-100ms blips, like someone suggested.  (I do not want
to use interrupts to drive a debug device, especially since TMR0 is
already used.)  So I'm not much into the idea of having the target
generate a carrier and modulation.  (That's a job for a 12c508..
anyone want to build a quick & dirty audible logic probe?)  1-2Hz
modulation seems a little slow, especially for debugging a state
machine with 8-16 states!  That's a lot of time to sit there counting
buzzes.

       It beats an LED for debugging!

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